The present invention relates to an exhaust gas routing device for an internal-combustion engine having a thermoelectric generator.
For the purpose of saving energy in the case of vehicles having internal-combustion engines, it has become known to provide thermoelectric generators in the exhaust gas line, which thermoelectric generators convert thermal energy to electric energy. The thus obtained electric energy can then, for example, be used for operating electric drives in the vehicle. Fuel can be saved in this manner because the electric energy does not have to be made available by means of the generating action of the internal-combustion engine.
German Patent document DE 10 2005 005 077 A1 discloses a thermoelectric generator for an internal-combustion engine. In the case of this known thermoelectric generator, damage to the generator elements is to be avoided in that the latter are arranged to be movable relative to a cooling mechanism and to a sleeve.
On the basis of U.S. Pat. Document US PS 960,324, a thermoelectric generator module has become known, which is to be suitable particularly for use at high temperatures.
Finally, the assignee's own German Patent document DE 10 2006 019 282 A1 describes an exhaust gas recirculation system for internal-combustion engines. Although this known exhaust gas recirculation system and the thermoelectric generator provided there have already had very good results, there is still space for improvement with respect to enhancing the performance of the thermoelectric generator.
Based on the above, there is therefore needed a device for routing exhaust gas for an internal-combustion engine by way of a thermoelectric generator that has a better performance characteristic.
This and other needs are met by an exhaust gas routing device for an internal-combustion engine having a thermoelectric generator fluidically connected to an exhaust gas line and with a line guiding a coolant, wherein the generator has a plurality of thermoelectric yoke pairs arranged between a first surface heated by the exhaust gas and a second surface cooled by the coolant. The exhaust gas routing device has a device guiding the exhaust gas along the first surface at a flow velocity increasing in the flow direction of the exhaust gas.
The invention is based on the recognition that the power yield of a thermoelectric generator can be improved if the individual thermoelectric yoke pairs of the generator each yield a largely equal and high electric power. The power of the thermoelectric yoke pairs is, among other things, a function of the temperature difference between the hot side and the cold side of the yoke pair.
At the location at which the hot exhaust gas enters into an exhaust gas routing device having the thermoelectric generator, the temperature of the exhaust gas will be higher than at the outlet side because the exhaust gas yields heat during its passage through the exhaust gas routing device, and a temperature gradient therefore exists from the inlet side to the outlet side.
When using thermoelectric yoke pairs of the same type in the flow direction, increased electric power could be achieved in an ideal condition if the temperature difference between the hot and cold side of the thermoelectric yoke pairs were to remain the same along the flow direction of the exhaust gas through the exhaust gas routing device. Also, any misalignment of the thermoelectric yoke pairs arranged along the flow direction has to be avoided.
As a function of the temperature difference, the yoke geometry and the thermoelectric material, a thermoelectric yoke pair has a characteristic electric current for achieving maximal recuperated electric power. Thus, a serial electrical circuiting of all yoke pairs, which is necessary for achieving a usable electric voltage, as a result of the changing temperature gradient in the flow direction in the case of a conventional construction, leads to a suboptimal electric current in the case of the plurality of the thermoelectric yoke pairs and therefore to a reduced power.
A temperature difference that remains the same along the entire exhaust gas routing device between the inlet side and the outlet side cannot be generated in the real operation of an exhaust gas routing device of the above-mentioned type. The invention solves this problem in a simple and efficient manner in that the device guides the exhaust gas along the first surface at a flow velocity that increases in the flow direction of the exhaust gas.
As a result, it is achieved that a lower flow velocity of the exhaust gas exists at the inlet side than at the outlet side. However, the exhaust gas temperature is also higher at the inlet side than at the outlet side.
By way of the lower flow velocity prevailing at the inlet side, a predefined heat transfer is achieved from the first hot surface to the thermoelectric yoke pairs arranged there. The exhaust gas dissipates heat when passing through the exhaust gas routing device, so that the temperature difference between the hot side and the cold side of the thermoelectric yoke pairs would decrease when it is assumed that a temperature of the coolant remains largely the same.
However, the flow velocity of the exhaust gas in the flow direction will increase simultaneously as a result of the device provided according to the invention, whereby the heat transfer in the flow direction of the exhaust gas can again be improved, and the negative effect of a temperature gradient between the inlet side and the outlet side of the gas routing device can be compensated.
In this manner, the temperature difference between the hot side and the cold side of the respective thermoelectric yoke pairs can be evened out along the flow direction through the exhaust gas routing device and thereby also the electric power of the thermoelectric yoke pairs, so that the problem of an electric misalignment of the thermoelectric leg yokes is also eliminated and the electric power as a whole that can be recuperated from the thermoelectric yoke pairs can be increased.
According to a further development of the invention, it is provided that the thermoelectric yoke pairs are arranged on a piping provided in the flow-through direction of the exhaust gas routing device. The device concentrically surrounds the piping. In the case of such an embodiment, known circular-disk-shaped thermoelectric yoke pairs can be used which are, for example, inserted into the annular gap between concentrically arranged pipes. In such an embodiment, the coolant can flow through the interior pipe according to the invention, so that the cooled second surface is situated radially on the inside, and hot exhaust gas flows around the exterior pipe on its outer circumference or the outer circumferential jacket surface, and the first surface heated by the exhaust gas is therefore situated radially on the outside.
Concentrically to the piping formed by the radially interior or radially exterior pipe, the above-mentioned device is provided which causes the flow velocity of the exhaust gas to rise in the flow direction of the exhaust gas from the inlet side of the exhaust gas routing device toward the outlet side. The increase of the flow velocity in the flow direction is achieved, for example, by an annular gap space between the device and the piping, thus, for example, the above-mentioned radially exterior piping, which annular gap space becomes smaller in the flow direction of the exhaust gas through the exhaust gas routing device.
Thus, the annular gap surface between the piping and the concentric device rises upstream toward the inlet side of the exhaust gas routing device according to an embodiment of the invention, which leads to an enlargement of the flow cross-section. When the flow cross-section in the area of the inlet side is compared to a flow cross-section of a known exhaust gas routing device, the thermoelectric generator of the inventive exhaust gas routing device has an electric power at the inlet that corresponds to that of the known exhaust gas routing device. However, the electric power of the known exhaust gas routing device with the flow cross-section that remains the same clearly decreases in the flow direction.
In the flow direction, a temperature gradient occurs between the inlet side and the outlet side. This temperature gradient leads to a lower temperature in the area of the outlet side of the exhaust gas routing device. Such a temperature gradient would result in a power drop of the thermoelectric yoke pairs in the flow direction, which, in the case of the exhaust gas routing device according to the invention, can, however, be compensated in that the flow cross-section of the device decreases in the flow-through direction of the exhaust gas guiding device and the flow velocity therefore rises. In view of the prevailing turbulent flow of the exhaust gas in the annular gap, this rise leads to an improvement of the heat transfer toward the hot side of the thermoelectric yoke pairs and thus to increased electric power.
The above-mentioned effect of the exhaust gas routing device can be achieved by the truncated configuration of the device provided according to the invention, which device concentrically surrounds the piping and ensures that the flow velocity of the exhaust gas increases in the flow direction.
According to a further development of the invention, a plurality of first and second surfaces arranged concentrically to one another is arranged in a housing having the exhaust gas routing devices for improving the electric power. In this case, the first surfaces heated by the exhaust gas are each surrounded by devices which are constructed in a truncated or conical fashion extending from the inlet side of the housing toward the outlet side and ensure an increase of the flow velocity of the exhaust gas relative to the first surface and thereby an improvement of the electric power of the thermoelectric generator as a whole.
The exhaust gas routing device according to the invention may be part of an exhaust gas recirculation system for an internal-combustion engine of a vehicle. It may have an exhaust gas cooler downstream which receives the exhaust gas cooled by the exhaust gas routing device and is fluidically connected with a fresh-air line of the internal-combustion engine for recirculating the exhaust gas for a reduction of pollutant emissions; i.e. the exhaust gas routing device can be integrated in an exhaust gas recirculation system.
The exhaust gas routing device according to the invention is also characterized in that an improvement of the electric power of the thermoelectric generator is achieved such that the second surface cooled by the coolant is situated radially on the inside and not, as in known arrangements, radially on the outside. The hot surface, around which the exhaust gas flows, for the heat transfer to the thermoelectric yoke pairs, is situated radially on the outside in this case, and correspondingly the surface cooled by the coolant is situated radially on the inside.
Other objects, advantages and novel features of the present invention will become apparent from the following detailed description of one or more preferred embodiments when considered in conjunction with the accompanying drawings.